linux-kernel - Re: [REGRESSION] threaded interrupt handler support breaks (some) irq handling on AVR32

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Date:	Wed, 15 Apr 2009 10:33:52 +0200
From:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
To:	Ben Nizette <bn@...sdigital.com>
Cc:	tglx@...utronix.de, David Brownell <david-b@...bell.net>,
	kernel@...32linux.org, linux-kernel@...r.kernel.org,
	imre.deak@...ia.com, mingo@...e.hu
Subject: Re: [REGRESSION] threaded interrupt handler support breaks (some)
 irq handling on AVR32

Haavard Skinnemoen wrote:
> Ben Nizette wrote:
> > static irqreturn_t ads7846_irq(int irq, void *handle)
> > {
> > 	struct ads7846 *ts = handle;
> > 	unsigned long flags;
> > 
> > 	spin_lock_irqsave(&ts->lock, flags);
> > 	if (likely(get_pendown_state(ts))) {
> > 		if (!ts->irq_disabled) {
> > 			/* The ARM do_simple_IRQ() dispatcher doesn't act
> > 			 * like the other dispatchers:  it will report IRQs
> > 			 * even after they've been disabled.  We work around
> > 			 * that here.  (The "generic irq" framework may help...)
> > 			 */
> > 			ts->irq_disabled = 1;
> > 			disable_irq(ts->spi->irq);  
> 
> Shouldn't that be disable_irq_nosync()?

Ok, simply stating that without providing an explanation was probably
not so helpful...

I think the problem is that disable_irq() calls synchronize_irq(),
which waits until the corresponding interrupt handler is no longer
running on any CPU. And since we're calling it from the interrupt
handler, we'll have a deadlock because the interrupt handler won't
return until synchronize_irq() returns, and that won't happen until the
interrupt handler returns and so on.

So I think changing disable_irq() into disable_irq_nosync() (which
doesn't call synchronize_irq()) will fix it, though I'm not sure what
difference the threaded interrupt handling code makes -- AFAICT, this
code has always been dangerous.

Haavard
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